Manufacture of cellulosic products



Oct. 31, 1961 Filed Dec. 15, 1957 J. W. DE VOS ETAL MANUFACTURE OF CELLULOSIC PRODUCTS 4 Sheets-Sheet 1 COOKING LIQUOR M02503 1 q Na s DIGESTER NQCOB BLOW TANK WASTE BRov/N STOCK 13 s WASHER N020 FLY MULTI-EFFECT 14 EvAPoRAToR s N020 1s j SMELTER 15 SEPARATOR 6A5 FURNACE NuzS N CO;

SMELT 1a 4 DISSOLVER 17 CLARIFIER wAsH WATER CRYSTALLIZING 1s SHELTER EvARoRAToR ms N,co (so| m) Nu C03(S0LN.) MOTHER uouoR HLTER WASHER "0 Na co ""z 2| DISSOLVER SUPPLY TANK so RECYCLE M120 uouoR SHELTER GAS 23 A WASTE GAS SCRUBBER 3 M12503 "5 NnzC0 4 SULFLR MAKE-UP 25 2 SULFUR 24 ABSORBER BURNER 4 AIR M11503 NaH$ 3 NaOH Na ca, FINISHING TANK 1 3 Na co Oct. 31, 1961 Filed Dec. 13, 1957 J. w. DE'VOS EI'AL 3 MANUFACTURE OF CELLULOSIC PRODUCTS 4 Sheets-Sheet 2 cooxmc LIOUOR "c 80 1 q. Na

1' N02C03 DIGESTER 12 now TANK WASTE BROWN smcx 13 S WASHER M120 s Na o SIDE STREAM MULTl-EFFECT 14 FLY ASH S EVAPORATOR N o 5 f M120 SPRAY rowan 26 OFF GAS S'MELTER 1s SHELTER 15A REACTOR s FURNACE GAS EPARAToR Na C0 ""2 Na CO REACTOR 27 SMELT I6 I DISSOLVER I DISSOLVER CARBONATE 2a WATER 17 CLAR'HER CLARIFIER wAsn CRYSTALLIZING 18 EVAPORATOR "a2:

Nu COjSOLID) Nu C0 (S0LN.) SHELTER MOTHER LIQUOR FILTER & 19 GAS WASHER Nu C0 CARBONATE 21 DISSOLVER Na CO 22 SUPPLY TANK 4 RECYCLE N 0 3 uouon so GAS SMELTER GAS ""2 s SCRUBBER "L1 0 2 3 NazCOg A SULFUR MAKE-UP 25 so, SULFUR 24 ABSORBER BURNER AIR a Na $0 ""2 20 NaOH Nu co FINISHING TANK Na S0 NazS NAIZCO3 Oct. 31, 1961 J. W. DE VOS EI'AL Filed Dec. 13, 1957 4 Sheets-Sheet 3 COOKING LIQUOR I 1 V z a DIGESTER 12 51.011 TANK WASTE BROWN new 13 S WASHER No. 0 s

N020 MULTI-EFFECT 14 4 FLY ASH EVAPORATOR SMELTER 1s SMELTER 15A FURNACE GAS SEPARATOR N025 Na ca;

SMELT 16 DISSOLVER SIDE STREAM 17 CLARIFIER N025 N G Na CO i a CRYSTALLIZING 1s EVAPORATOR H1125 Na C0 (SOL|D) SMELTER WTHER QUOR Nu C0 (S0LN.) GAS FILTER & 19

WASHER 11-80 zlNc CYCLE 29 1,2503 REACTORS M12803 DISSOLVER ZnS z1Nc CYCLE 3o FILTER M12503 Na CO 22 Zn: SUPPLY TANK 2 mm CYCLE 31 MR M02503 5222:

ROASTER. WASTE ""2 Z GAS SHELTER GAS 23 "2 5 SCRUBBER z1Nc CYCLE 32 M1120 "c 50 ABSORBER co l SULFUR MAKE-UP 2s SULFUR 24 ABSORBER BURNER AIR "0 50; Na S NuHS0 N OH FINISHING TANK g g fl Na ca;

Oct. 31, 1961 J. W. DE VOS ETAL MANUFACTURE OF CELLULOSIC PRODUCTS Filed Dec. 13, 1957 4 Sheets-Sheet 4 COOKING LIQUOR I "c 50 l7 ""2 n Nn co DIGESTER 12 51.0w TANK wAsTE BROWN srocx 13 s WASHER M120 S FLY ASH MULTl-EFFECT 14 r EVAPORATOR 15A MELTER SMELTER 1s SEPARATOR GAS FURNACE N025 Na CO: SMELT 16 SHELTER DISSOLVER GAS N025 U Nu CO CLARIFIER WASTE as r ZINC CYCLE 32 ZINC CYCLE 29 1 ABSORBER REACTOR M02303 Na CO ZnS AIR ZINC CYCLE 31 W zmc CYCLE FILTER N'u SO SIDE 'z s STREAM 22 SUPPLY TANK $02 RECYCLE "n20 LIQUOR TE 23 22555852.? v

M02503 N020 co SULFUR MAKE-UP 2s SULFUR 24 MR ABSORBER BURNER "0 50 "no" NuHSO NGZS 20 FINISHING TANK 4 N o N05 3 Nu CO3 United States Patent l 3 6, 04 MANUFACTURE OF CELLULOSIQ PRUDUCTS John Waiiace de Vos and Harry B. Kellogg, Apple ton, VVisr, assignors to Kimberly-Clark Corporation, Nee: nah, Wis., a corporation of Delaware Filed Dec. 13, 1957', Sci. No. 702,722 5 Claims. (Cl. 152'36) The present invention is concerned with the manufacture of cellulosic products and more particularly with a process for the recovery of chemicals from the spent liquor from a wood pulping method and the economical reconstitution of fresh pulping liquor from the recovered chemicals.

Wood and other celluiosic raw materials may be converted into pulp which is suitable for the manufacture of paper and similar paper products by the treatment of wood at moderate temperatures and pressures with a chemical solution usually referred to as a pulping or cooking liquor. There are numerous chemical compositions which may be used to make up the cooking liquors and these may be divided into two general types, the acid liquor and the alkaline liquor.

The chemicals used to make up alkaline cooking liquors are normally compositions of sodium and/or sulfur. The chemical constituents of the cooking liquor which are effective in converting the wood into pulp are usually referred to as active chemicals. In the alkaline process typical active chemicals are sodium hydroxide; sodium sulfide and sodium sulfite. Now; in addition to the active chemicals present in the cooking liquor, other chemicals which are non-functional in the pulping process, may also be present in the cooking liquor. Sodium carbonate, sodium sulfate and sodium thiosulfate are examples of these non-functional chemicalswhich may be present in alkaline cooking liquors. These non-functional chemicals are usually referred to as inactive chemicals. The spent cooking liquor, which is separated from the wool pulp after completion of the digestion of the wood, contains any excess active chemicals over that used in converting thewood into pulp, the inactive chemicals and the reaction products of the pulping operation. The reactions which take place in the digester involve solubilizing the lignin and release of the cellulose. Salts such as sodium lignosulfonates which are soluble in water under alkaline conditions are formed. Simultaneously, most of the extracted fats, resins, etc., are saponified anddiss'olved in the cooking liquor. Because of the complex nature of the chemical substances in spent cooking li'qu'or the individual components are not usuallyidentified but the elemental content such as the sulfurcontent, is usually specified. In the case of sodium, the content is 'peeifi'ed as the sodium oxide, Na O. The sulfur and sodium components of spent cooking liquors are valuable and it is highly desirable that these components be recovered in such form that they can be readily converted into the active chemical compounds which are used to make up a fresh cooking liquor for subsequent pulping operations. In order to most effectively utilize the chemicals recovered from spent cooking liquor it is desirable that the components not only are converted into active chemical components which can be used to make up fresh cooking liquor but also that the components are recovered in the proper proportions to make up the fresh liquor.

Numerous recovery processes have been invented for conventional types of alkaline pulping processes such as the kraft process and the soda process. The kraft recovery processes are designed to convert the spent liquor from kraft digestions into a fresh liquor containing sodium hydroxide and sodium sulfide in the proper propor tions while minimizing the formation of sodium car- 3,006,804 Patented Oct. 31, 19 61 bonate, sodium sulfate and sodium thi'osulfate. The recovery processes for spent soda process-liquor are designed to produce a fresh liquor consisting essentially of an aqueous solution of sodium hydroxide. The pres ent invention is concerned with the recovery process for a sodium mon'osulfite-sodium sulfide type liquor such as is used in the co-pendmg United States patentapplication Serial No. 680,701, of Ralph V. Braun, filed August 28, 1957. The active chemical in the cooking liquor of the Brauh pulping process consists of about 15 to 30 'percent by weight of the active chemical of sodium sulfide and to 70 percent by weight of sodium sulfite. The ratio of active chemical (sodium sulfide and sodium sulfite) to cooking liquor is about -130 parts by weight 'of active chemical per 1000 parts by weight of cooking liquor. Sodium carbonate is a non functional chemical in the Braun process and may be tolerated in the cooking liquor in small amounts, pref erablyless than about v12 percent by weight of the total chemical content. Sodium thiosulfate may have a deleterious eifect on the bleachability of the pulp produced and should be limited to less than about 5 percent. Sodium hydroxide and sodium sulfate are detrimentalin the process and the addition of these chemicals to the cooking liquor is avoided.

Other pulping processes employing sodium monosulfite as a pulping agent have been employed in the past. Although the sodium base sulfite chemicals employed in these processes are relatively expensive and the chemical consumption in these processes are high the pulp producted has very outstanding characteristics. The bleachability of the pulp is approximately equivalent to the best acid sulfite pulp and the strength is as good as that of kraf t pulp. As pointedout by Casey, Pulp and Paper, volume I, Interscience, 1952, page 215, there has been no effective recovery system for the spentrnonosulfite pulping liquors and this is an economic necessity in view of the high chemical demand of the pulping process and the cost of the sodium base hquors. The smelting of a monosulfite spent liquor in a conventional manner will result in the production of a substantial amount of sodium sulfide. When a sodium sulfidecontaining hquor istreated with sulfur dioxide or sulfited as it is usually called a substantial proportion of the sulfide in the liquor is converted. into thiosulfate and. as previously pointed out only a very small amount ofthiosulfate can be tolerated in a pulping process. The sulfitation of the sulfide containing liquor also produces hydrogen sulfide, the fumes of which are both noxious and poisonous. The smelt from a monosulfite pulping liquor may be used as make up for a kraft process typeliquor but this would necessitate operating the sulfite type pulping process in conjunction with a kraft type pulping process and thus limit the commercial feasibility of the sulfite process.

It is an object of the present invention to provide a method for the recovery of the sulfur and sodium content from spent pulp cooking liquors of the character described. I

It is an additional object of the present invention to provide a method for the recovery of a spent sodium monosiilfitecooking liquor in which the mono'su lfite is recovered by a' sulfitation step in the absence of sulfide.

It is a further object of the present invention to provide a method for converting the sodium and sulfur contents of spent monosuliite cooking liquors into sodium sulfide and sodium sulfite in the proportions suitable for use in a monosulfite cooking liquor.

It is a further object to provide a method for the conversion of a spent monosulfite cooking liquor into a finished cooking liquor which minimizes the production solved smelt in several ways.

of sodium carbonate, sodium sulfate and sodium thiosulfate. 7

Other objects will be apparent from the description of the invention which follows:

In the drawings;

FIGURE 1 is a flow diagram of the preferred embodiment of the present invention.

FIGURE 2 is a flow diagram of a second embodiment of the present invention.

, FIGURE 3 is a flow diagram of a third embodiment of the present invention.

FIGURE 4 is a flow diagram of a fourth embodiment of the present invention.

I11 3.CCOI'(iaI1CB with the process of the present invention it has been found that a concentrated sodium mono sulfite-sodium sulfide spent pulping liquor can be processed by burning the concentrated liquor to form a solids fraction containing sodium sulfide and sodium carbonate and a gaseous fraction containing sulfur dioxide. The

solids fraction (which is molten at the furnace temperature) is then dissolved in an aqueous phase. A sulfide portion containing a predetermined amount of sodium sulfide is divided out from the dissolved solids fraction. The sulfide portion may be divided out from the dis- If the total sulfide in the dissolved smelt is equal to the predetermined amount required in the reconstituted liquor, it is divided out by evaporating the smelt solution to crystallize out essentially pure sodium carbonate and then by filtering or centn'fuging the crystal suspension, separating the filtrate from the crystals. All the sodium sulfide and some carbonate will remain in solution in the filtrate which is sent directly to the finishing tank for incorporation in the reconstituted liquor. If the sodium sulfide in the dissolved smeltis in excess of the desired predetermined amount, the smelt solution is divided into two portions. One portion, containing the desired amount of sodium sulfide, has the major amount of sodium carbonate removed by crystallization and the filtrate is sent directly to the finishing tank for incorporation into new cooking liquor. The second portion of the smelt solution, containing excess sodium sulfide, is treated in one of the ways hereinafter described to convert the undesirable excess sulfide to sodium sulfite or sodium carbonate. The latter is then combined with the substantially pure sodium carbonate solution obtained from the'crystallization step. The combined sodium carbonate is sulfite with the sulfur dioxide of the gaseous fraction to form a sulfite portion. Losses of sodium sulfite throughout the pulping-recovery cycle may be compensated for by the addition of sodium ions and sulfite ions to the sulfite portion. The sulfite and sulfide portions are then combined to form a reconstituted sodium monosulfite-sodium sulfide pulping liquor.

The process of the present invention is particularly applicable to the recovery and reconstitution of waste liquors resulting from a pulping operation employing a liquor such as is described in the Braun co-pending application. Such a liquor contains approximately 60-80 percent sodium sulfiteand 3010 percent sodium sulfide and not more than about 12 percent inactive chemical. The inactive chemical is predominately sodium carbonate but sodium thiosulfate to the extent of about percent of the total chemical content of the liquor may be present. The concentration of the active chemical is usually about 90-130 parts of active chemical per 1000 parts of liquor. The moisture content of the wood is included in this concentration. The spent liquor recovery process is initiated by separating the spent liquor from the pulp and evaporating the spent liquor to a solids content of about 50-75 percent. The concentrated liquor is burned so that a solids fraction containing sodium sulfide and sodium carbonate and a gaseous fraction containing sulfur dioxide is formed. This molten solids fraction is dissolved in an aqueous phase. A sulfide portion containing sodium sul- 4? fide in an amount equivalent to that present in the original pulping liquor, and containing not more than about 12 percent inactive chemical is divided out from the aqueous solution of the solids fraction either by splitting the main stream into one containing the predetermined amount of sulfide and a second stream containing the remainder, or by crystallizing out the sodium carbonate from the entire main stream when said stream contains the proper predetermined amount of sulfide. Any excess sodium sulfide is left with the secondary stream of dissolved solids fraction and is converted into sodium carbonate or sodium sulfite by one of the methods hereinafter described and combined with sodium carbonate of the solids fraction obtained by crystallization. This combined sodium carbonate is then sulfited with the sulfur dioxide of the gaseous phase to form a sulfite portion. Sufiicient sodium ion and sulfite ion are added to the sulfite portion to increase the sodium sulfite content of said portion to about 60-80 percent of the chemical content of the original pulping agent. The sulfite and sulfide portions are then combined in an aqueous liquor containing about 6080 percent sodium sulfite and 30-10 percent sodium sulfide and not more than about 12 percent inactive chemical. The concentration of the active chemical may be adjusted to any convenient ratio by the addition of water or by evaporation. In the Braun liquor it is about 90l30 parts of active chemical per 1000 parts of liquor (including the moisture content of the wood). Many of the steps of the present process can be carried out in existing chemical equipment such as is used in the kraft type pulping processes.

Where the present process is used to recover monosu-lfite liquor from a batch type pulping operation upon completion of the digestion the pulp and liquor may be blown from a digester and separated from the steam and fumes in blow tank such as is conventionally used. The pulp may be separated from the spent pulping liquor and washed in conventional brown stock washers. The monosulfite liquor may either be concentrated to a high solids content in conventional multi-effect forced circulation evaporators such as are used to concentrate kraft waste liquors or may be concentrated by using a natural circulation evaporator, preferably of the 'multi-effect type. These concentration steps are, however, carriedout in the conventional manner and may be carried out with conventional equipment such as is used in the recovery of kraft type liquors.

An important attribute of the present invention is its adaptability to various types of existing'combustion equipment; The burning of the concentrated waste liquor in the present process may be carried out in several different types of combustion equipment by modifications of the process which will be described. These modifications of the present process are particularly based upon the ratio of sodium sulfide to sodium carbonate which is produced in the combustion step. It is essential in the present process that any excess sodium sulfide produced in the combustion step over that amount required for the reconstituted liquor be converted prior to the sulfitation step into a recoverable salt which will not produce hydrogen sulfide, or other harmful products during the sulfitation step. Sodium carbonate, sodium bicarbonate and sodium sulfite are suitable salts. Four'embodiments of the present invention are described and illustrated in the figures.

In the preferred embodiment illustrated in FIGURE 1 the combustion may be carried out in the type of furnace ordinarily used for the recovery of kraft type cooking liquors in which the method of operation of the furnace is altered so as to produce a smelt having a ratio of sodium sulfide to sodium carbonate of about 1 to 2.5. In the second and third embodiments of the present invention, illustrated in FIGURES 2 and 3, the combustion step may be carried out in a typical kraft recovery type furnace, operated in a conventional manner so that the sulfide to carbonate ratio of the smelt is about 1 to 1. In the second embodiment the excess sulfide is converted to the carbonate by another parallel process in which the excess sulfide is converted into the carbonate. In the third embodiment the sulfide is converted to the carbonate by a metathesis reaction carried out in an aqueous phase. In the fourth embodiment illustrated in FIGURE 4 the combustion step may be carried out in a combustion furnace operated in such a manner as to produce a sulfide to carbonate ratio of about 2 to 1. In this embodiment the excess sulfide is converted to carbonate by an aqueous phase reaction. The smelt dissolution step and clarification in all modifications of the present process are carried out in substantially the same manner as in kraft recovery processes. It will be noted as the various embodiments are presented that in all embodiments the sulfitation is carried out in aqueous, sulfide-free solutions. It will also be noted that substantially all steps of the present process may be carried out in either existing equipment presently used in the recovery of kraft type liquors or in conventional chemical apparatus.

In the preferred embodiment of the present invention, as illustrated diagrammatically in FIGURE 1, the spent liquor and pulp are discharged from the digester 11 upon completion of the digestion period into blow tank 12 as in conventional pulping practice. Sulfur is present in the spent liquor in the form of various organic and inorganic compounds such as the ligno-sulfonates and sulfite and sulfide compounds. The sodium is also present as complex sodium salts and the sodium content of the liquor is usually described in terms of the sodium oxide (Na O) The steam is separated from the pulp and spent liquor in the blow tank 12 and the pulp and liquor are then transferred to the brown stock washer 13. While this may be a single stage washer it is more commonly a multiple stage washer by which the pulp is washed free of as much of the spent liquor as is economically feasible. Conventional brown stock washers such as are used in the kraft process may be employed. In the present recovery process there is some loss of sulfur content and sodium content in this washing operation and this may amount to about 2 percent each of the sulfur and sodium contents of the spent liquor.

The spent cooking liquor after its separation from the pulp in the brown stock washer is then concentrated in the evaporator 14. The purpose of the evaporator is to concentrate the liquor to such a density that it can support combustion when sprayed into a smelter furnace. The concentration of solids in the liquor as it is removed from the brown stock washer as it is usually called is ordinarily about 17 percent. are used in the kraft process and are described in Pulp and Paper Manufacture, volume 1, Stevenson, 1950, may be used to effect the evaporation of the black liquor in the present process. the present process is somewhat less viscous than the spent liquor from the kraft process and thus may be either evaporated to a higher solids content than kraft liquor in conventional forced circulation evaporators of the type used for kraft liquor or evaporated to the same density in natural circulation evaporators. Thus the black liquor may be evaporated from an original density of 14 to 18 percent total solids to between about 50-75 percent total solids, depending upon the type of recovery unit used. As will be pointed out in connection with the description of the second embodiment, the concentration of the black liquor may in some cases be carried out as a part of the combustion step.

In the conventional kraft process the concentrated black liquor is sprayed into a furnace where it is burned to remove the organic components. There are several different types of furnace which are conventionally used such as are described in chapter 5, Pulp and Paper Manufacture, ibid. In a typical furnace of the Tomlinson- Standard evaporators such as The spent liquor or black liquor of Babcock Wilcox type the concentrated black liquor is sprayed into the furnace about midway between the top and bottom of the furnace so that the spray strikes the opposite wall of the furnace. The spray is dehydrated by its passage through the hot furnace gases and contact with heated walls of the furnace and as it or its derived solids accumulates upon the walls it falls of its own weight to the hearth of the furnace where it is allowed to accumu late in the char bed, the depth of which may be varied. Primary air ports admit air near the base of the furnace thus burning out the carbonaceous material originally contained in the black liquor. There are secondary air ports part way up the walls of the furnace which admit air above the char bed and thus complete the combustion of the gases before they leave the furnace. If a concentrated spent monosulfite cooking liquor is burned in a conventional ltraft recovery furnace operated in the usual man- 'ner the products of the char bed are sodium sulfide and sodium carbonate in about a l to 1 ratio. In the conventional recovery system for kraft liquor, a deep bed of char is allowed to accumulate in the furnace which promotes the reaction:

N32SO4+2C9 Na S+2CO (1) The sodium sulfide is most readily produced in a recovery process in the combustion furnace. Now in the present process it is desirable to limit the amount of sodium sulfide produced to that amount that can be reused in the reconstituted liquor. Any excess of sulfide must be converted into other sodium salts before the sulfitation step of the process in order to avoid thiosulfate and hy drogen sulfide production. It will be noted that hydrogen sulfide is produced in the furnace but this is almost hnmediately oxidized to sulfur dioxide and thus is not harmful. It is therefore desirable when the present invention is used to produce a Braun type liquor, to operate the furnace 14 so as to obtain a smelt containing sodium sulfide and sodium carbonate in a molar ratio of about 1 to 1.45 to 1 to 3.5 or 1 to 2.3 to 1 to 5.7 by weight ratio. This may be accomplished by using a furnace designed to produce this ratio or it may be accomplished by operating a kraft type liquor recovery furnace in an unconventional manner. If it is carried out in a kraft type furnace the depth of the char bed is reduced so as to minimize the production of sodium sulfide. The primary air supply, however, is increased so as to promote Reactions 2 and 3 above and the oxidation of the carbon present to the dioxide. The furnace can thus be regulated to produce the desired smelt ratio.

The smelter gas is transmitted through heat exchangers (not shown) and a separator 15a and finally to the smelter gas scrubber 23 where it is reacted with sodium carbonate. The heat exchangers may be substantially the same as those in any conventional kraft recovery system. The separator 15a may be a conventional separator for separating solids from gases such as the centrifugal type or an electrostatic precipitator or a combination of these. The solids or fiy ash separated from the gas is returned to the incoming black liquor from the multi-eifect evaporator 14. V

The smelt from the furnace 15 is permitted to overflow the solution to a smelter gas scrubber 23.

i sulfide and 2.3 to 5.7 parts by weight of sodium carbonate,

is then conducted to a crystallizing evaporator 18. Sodium carbonate is less soluble than sodium sulfide and the carbonate may be separated from the sulfide by evaporating a portion of the green liquor solution so that most of the carbonate crystallizes out of the solution. This may be accomplished in any suitable crystallizing evaporator such as those described in Chemical Engineers Handbook, Perry, 2nd ed., page 1779. Since the pulping operation can tolerate a certain amount of inactive chemical in the digestion of the cellulosic raw materials and it is uneconomical to completely separate the sodium sulfide and sodiumvcarbonate by crystallization, a small portion of sodium carbonate, about or 12 percent is usually permitted to remain with the sodium sulfide in the mother liquor. The mother liquor is separated fromthe crystals of sodium carbonate in the filter and Washer 19. The mother liquor contains the desired amount of sodium sulfide for the finished liquor and it is transferred directly to the finishing tank 20. The wash water may be returned to the smelt dissolver 16.

Following the filtering of the sodium carbonate crystals from the mother liquor, these are washed in the filter and washer 19 and then dissolved to form an aqueous solution of sodium carbonate in the dissolver 21. This solution is conveyed to a supply tank 22 which supplies In the smelter gas scrubber the sodium carbonate solution is used to scrub the sulfur dioxide from the smelter gas from the smelter furnace 15. This smelter gas scrubber may be a conventional liquid-gas contacting apparatus such as a packed tower, wetted column, etc. Since the filtered crystalline sodium carbonate which had been dissolved to make the aqueous carbonate solution used in the scrubber is substantially freed of sochum sulfide no sodium thiosulfate or hydrogen sulfide is formed in the scrubbing operation. Modifications of the scrubbing operation may be made in accordance with established engineering practice. For example, in the first embodiment where there is a comparatively large quantity of smelter gas it may be desirable to recycle a portion of the liquid effluent from the scrubber 23 to the supply tank 22 so that there is at all times an adequate amount of liquor for effective scrubbing. The waste gases which may contain as much as 16% of the sulfur present in the smelter gas may be emausted to the atmosphere preferably through tall stacks since it is very largely diluted with inert gases.

The aqueous product of the scrubber is a solution containing predominately sodium sulfite with a small amount of sodium carbonate. While we do not Wish to be bound by any theories advanced it is believed that the reaction which takes place may be represented by the following equations:

The equilibrium of Reaction 4 is such that at a pH of about 9 this reaction goes substantially to completion. The two reactions are cyclic in that the carbonate produced in Equation 5 reacts with fresh sulfur dioxide.

Since there are certain chemical losses in the pulpingrecovery system it is desirable to make up the sulfur losses by the addition of sulfur dioxide to the effluent solution from the scrubber. Sulfur may be burned in a conventional sulfur burner 24 to form sulfur dioxide and this added to the aqueous solution from the scrubber in a make-up absorber apparatus 25. This absorber may be a conventional vapor-liquid contactor which will, however, be much smaller than the smelter gas scrubber be- 5 cause of the smaller volume of gas handled. The sulfur dioxide is absorbed in the make-up absorber to produce a solution of sodium sulfite and sodium acid sulfite. Typical reactions which may occur in the make-up absorber are as follows:

Because of the lower pH, Equation 7 proceeds to some extent in the make-up absorber 25. The temporary formation or the sodium acid sulfite in the make-up absorber, however, tends to drive the carbon dioxide from the stream.

The solution or" sodium sulfite and sodium acid sulfite from the make-up absorber 25' is then treated with an 29 aqueous solution of sodium hydroxide. This solution of sodium hydroxide not only makes up any losses of sodium which occur throughout the recovery cycle but also neutralizes the sodium acid sulfite in accordance with the following reaction:

The stream of the mother liquor from the filter washer 1? containing the desired amount of sodium sulfide and some sodium carbonate is finally mixed with the sodium 0 sulfite solution in the finishing tank to form the completed liquor. The neutralization of the sodium acid sulfite is always carried out before the addition of the mother liquor to the sodium sulfite solution.

The process of the present invention may be modified to accommodate the process to various conditions. For example, it may be desirable to operate the process with a kraft type smelter furnace operating in the conventional r lner to produce sodium sulfide and sodium carbonate in approximately a l to 1 molar ratio (1 to 1.36 weight 49 ratio). This modification of the invention is illustrated in the flow diagram of FTGURE 2.

in this modification the digester 11, blow tank 12, brown st ck sa r and multi effect evaporator 14 are operin the same manner as shown in the first embodiment wur invention previously described. However, since melter furnace in this embodiment is operated in such a manner that it only produces approximately equivalent molar amounts of sodium carbonate and sodium sulfide it is necessary that the stream of black liquor from the 59 multi-eifect evaporator be separated so that only the amount of sodium sulfide desired in the final product will he produced in the smelter furnace 15. The sulfur content of the side strcamis then converted substantially completely into sulfur dioxide and any reaction which would result in sulfide production is avoided.

The conversion of the sulfur and sodium contents of waste liquor solids in the side stream into sodium carbonates and sulfur containing gases can be accomplished by the pyrolysis of the black liquor at elevated tempera- 5 tures of the order of l2GO-l800 F. in closed reactor. The sulfur containing gases can then be oxidized with air to sulfur dioxide in a separate reactor. The pyrolysis reaction on the side stream of black liquor may be carried out in a spray tower reactor 46. V

The main stream of the black liquor is directed to the smelter furnace 15. The main stream of black liquor is burned in the smelter furnace 15 in the conventional manner such as described in Pulp and Paper lslan ufacture, ibid., chapter 5, to produce the smelt containing approximately equal molar amounts of sodium sulfide and sodium carbonate. This smelt is dissolved in a smelt dissolver 1t? and clarified in a clarifier 17 substantially as described in the first modification of our invention. The smelter gas from the smelter furnace 15 is separated from the solid fly ash in a separator 15 and the fly ash returned to are evaporator 14 in the same manner as described in our first embodiment. The clarified smelt or green liquor from the clarifier i7 is then introduced into a crystallizer evap orator 18 where a substantial portion of the sodium carbonate is crystallized out by evaporation. The mother liquor and the crystals are separated in the cry tal washer 19 and the mother liquor which contains the desired quan tity of sodium sulfide for the make up of the finished liquor and also contains a small amount of sodium carbonate and other impurities is then introduced into the finishing tank 2 9. The sodium carbonate crystals are Washed in the filtering Washer 19 and are then dissolved in the carbonate dssolver 21 to form an aqueous sodium carbonate solution.

It is desired to convert the sodium and the sulfur contents of the side stream of black liquor from evaporator 14 into sodium carbonate and sulfur dioxide respectively. The side stream of liquor is therefore introduced into a spray type reactor, which is closed to the atmosphere and is heated to a high temperature by the application of external heat to the wall of the reactor. The heat may be obtained from the exhaust gases from the smelter. Under these conditions the finely divided black liquor is pyrolyzed to form solid sodium carbonate and a gas containing sulfur compounds predominately consisting of hydrogen sulfide and sulfur dioxide. The ofi gas from the spray tower reactor 26 is reacted with oxygen to insure the complete oxidation of the sufur in the gas to the sulfur dioxide. This may be carried out by introducing the off gas from the reactor 26 into the smelter furnace Where there is a sumcient supply of air at high temperatures which completely oxidizes sulfur content of the reactor oil gas to sulfur dioxide. The resultant sulfur dioxide mingles with the oil gas from the smelter furnace and is thus transmitted to the smelter gas scrubber 23.

The sodium carbonate from the spray tower reactor 26 can be dissolved and separated-from carbon and other insoluble material in a reactor dissolver 27 which can be of similar design to the smelt dissolver 16. The slurry from the reactor dissolver is clarified in a carbonate clari fier Z8 and the clarified carbonate solution is then united With the carbonate solution of the main stream in the supply tank 22. The remainder of the process of the present embodiment is identical with that of the first embodiment of our invention. The resultant product is a cooking liquor containing 60-80% by weight of sodium sulfite, 3()10% sodium sulfide and not more than about 12% inactive chemical.

The present embodiment may be varied to recover and reconstitute cooking liquors having various sulfide-sulfite ratios. For example, for liquor having the sodium sulfidesodium sulfite molar ratio of active chemical varied between 1 to 3.54 and 1 to 1.45 as it does in the Braun process, the smelt ratio of the combustion step could be varied between 1 to 1.65 and 1 to 0.34 in order to produce a solids fraction having the proper amount of sodium sulfide.

A third embodiment of the present invention is concerned with reconstituting an alkaline sulfite spent liquor where all of the concentrated spent liquor is reacted in a kraft type recovery furnace operated in the conventional manner so as to produce sodium sulfide and sodium carbonate in a molar ratio of about 1 to 1. The required amount of sodium sulfide is separated from the solids fraction and the balance of the sodium sulfide is converted into sodiurn carbonate by an aqueous phase metathesis reaction prior to the sulfitation step. The steps of the process are illustrated in the flow diagram of FIGURE 3.

In this alternative embodiment the entire stream of spent cooking liquor is removed from the digester into the blow tank 12, separated from the pulp in the brown stock washer 13, evaporated in the multi-effect evaporator 14 and concentrated in the same manner as is the stream of liquor in the first embodiment of the present invention. The concentrated liquor from the multi-efiect evaporator,

however, is burned in a conventional kraft type smelter furnace operated in a manner which will produce approximately a 1 to 1 molar ratio of sodium sulfide to sodium carbonate. The smelt from the furnace is dissolved in the smelt dissolver 16 and clarified in the clarifier 17 in a manner identical with that described in the first embodiment of the present invention. The clarified products stream, however, is split as it leaves the clarifier so that a main stream contains the amount of sodium sulfide desired in the finished cooking liquor and the balance of the sodium sulfide is contained in the side stream. Since the molar ratio of sodium sulfide and sodium carbonate is 1 to 1 in the product stream at this point the main stream and side stream will also contain sodium carbonate in this ratio. The main stream from the clarifier containing the desired amount of sodium sulfide and an equivalent amount of sodium carbonate is partially evaporated in a crystallizing evaporator to separate the sodium sulfide from a portion of the sodium carbonate. The resultant crystalline sodium carbonate is separated from the mother liquor and Washed in the filter and washer 19 and the mother liquor is transmitted to the finishing tank 29 in the same manner as these materials are treated in the first embodiment of the present invention.

In order to completely free the side stream of sulfide prior to the sulfitation step the sodium sulfide is reacted with zinc sulfite in an aqueous slurry. An exchange or metathesis reaction takes place according to the following equation:

The Zinc sulfide formed is very insoluble so that the reaction goes essentially to completion.

The side stream containing sodium sulfide and sodium carbonate is introduced into the zinc cycle reactor 29. There the side stream is reacted with an aqueous zinc sulfite slurry. The insoluble zinc sulfide thus formed is then separated from the soluble sodium sulfite and sodium carbonate in the zinc cycle filter 30 and the sodium sulfite-sodium carbonate stream then combined with the sodium carbonate solution of the main stream in the supply tank 22. The sulfitation step is then carried out as in the first embodiment using the sodium carbonatesodium sulfite solution contained in supply tank 22 as the scrubbing agent. The sodium carbonate is converted into sodium sulfite by reaction with the sulfur dioxide of the smelter gas. The fortification of the resultant sodium sulfite solution with sulfur and with sodium takes place in the make up absorber 25 and the finishing tank 20 as in the first embodiment. The solution is converted into a sodium sulfite solution in the smelter gas scrubber 23, fortified with the sulfur dioxide in the make up absorber 25 and with sodium hydroxide in the finishing tank 20 in the same manner as in the first embodiment of the present invention.

The zinc sulfide which is separated from the sodium sulfite and sodium carbonate in the side stream in the Zinc cycle filter 30 may be reconverted into zinc sulfite by first roasting the zinc sulfide with air to form solid zinc oxide and gaseous sulfur dioxide. The zinc oxide is suspended in water to form a zinc oxide slurry. The sulfur dioxide is then reacted with the zinc oxide slurry in a zinc cycle absorber 32 to form the zinc sulfite slurry and thus complete the zinc cycle.

As in the preceding embodiment the present modification can be varied to produce varied ratios of sodium sulfide-sodium sulfite in the finished liquor. Thus the furnace operation may be varied to produce difierent smelt ratios. Alternatively the amount of the side stream separated from the main stream can be increased or decreased to insure that the main stream contains the desired amount of sodium sulfide. Similarly the crystallization step may be varied to increase or decrease the amount of sodium carbonate permitted to enter the finished liquor.

sulfur and pound moles of sodium oxide (N320).

A fourth embodiment of the present invention is concerned with the operation of the process when the smelter furnace which is used to burn the concentrated liquor produces sodium sulfide and sodium carbonate in a molar ratio of about 2 to 1 or higher. In this modification a spent liquor is separated from the pulp and concentrated in the same manner as in the first embodiment of our invention. The concentrated liquor is burned in a smelter furnace in such a manner as to produce a mole ratio of sodium sulfide to sodium carbonate of about 2 or more to 1. This may be accomplished in a specially designed furnace or may be accomplished in a kraft type furnace by a suitable modification of the operating conditions. For example the char bed depth could be increased over that conventional in kraft recovery operations and the primary air to'the furnace decreased. The smelt from the furnace is dissolved in the dissolver 16 and clarified in the clarifier 17 in the same manner as the smelt is handled in the first embodiment. A side stream of liquor which contains the desired amount of sodium sulfide to make up a finished liquor may be taken directly from the clarifier. The sodium sulfide, sodium carbonate ratio is so high that the sodium carbonate introduced into the inished liquor with the side stream is usually present in an amount which can be tolerated in the pulping operation. For example, where the mole ratio of sodium sulfide to sodium carbonate produced in the smelter furnace is 2 to 1 and 29.6 pound moles of sodium sulfide are contained in the finished liquor the carbonate content of the side stream will be 14.8 pound moles of sodium carbonate.

The main stream from the clarifier can be immediately reacted with zinc sulfite in the same manner as was the side stream from the clarifier in the third embodiment of the present invention. Upon completion of the metathesis reaction in the zinc cycle reactor the resultant mixture may be filtered in a zinc cycle filter 3% to remove the solid zinc sulfide and the solution of sodium sulfite-sodium carbonate transmitted to the supply tank 22. The zinc sulfide is roasted in a Zinc cycle roaster 31; the resultant zinc oxide and sulfur dioxide reacted in a zinc cycle absorber '32 and the zinc sulfite is slurried with water to make up zinc sulfite slurry suitable for reuse in the zinc sulfite reactor 29.

The smelter gas is scrubbed in the smelter gas scrubber by the sodium sulfite-sodium carbonate solution from the supply tank 22. The main stream from the smelter gas scrubber is then fortified with sulfite dioxide in the make up absorber and with sodium hydroxide in the finishing tank 29 before it is united with the side stream from the clarifier 17 to form the finished liquor. The resultant liquor contains the desired ratio of sodium sulfide to sodium sulfite and not more than 12% of inactive chemical. Now that the process has been broadly described the process will be further illustrated with specific examples.

Example 1 The cooking liquor which was recovered in the present example initially contained 73.1 pound moles of sodium sulfite, 29.6 pound moles of sodium sulfide and 10 pound moles of inactive chemical, predominately sodium carbonate. The inactive chemical in all of the examples is treated as sodium carbonate. The concentration of the active chemical in the initial cooking liquor was 120 parts of active chemical per 1060 parts of liquor and the liquor to wood ratio was 5 to 1. Upon completion of the digestion, which was carried out in accordance with the method described in the co-pending Braun application previously referred to, the contents of the digester were blown to the blow tank 12 and the pulp then separated from the waste liquor in the brown stock washer 13. Because of the complex nature of the compounds in the liquor the sulfur content and the sodium content were determined and are expressed as pound moles of The liquor entering the brown stock washer was thus found to contain 102.7 pound moles of sulfur and 112.7 pound moles of sodium oxide. The losses in the brown stock washer were 2.2 pound moles of sulfur and 2.2 pound moles of sodium oxide.

The waste liquor from the washer was concentrated in thernulti-efiect evaporator 14 from a solids concentration of 17% by weight to a solids concentration of 60%. The concentrated liquor was then sprayed into a smelter furnace. The bed of char in the furnace was shallower than the normal bed of a kraft furnace and a substantially greater'amount of primary air was admitted then normally used in a kraft smelting operation. The smelt produced in this furnace contained 29.6 pound moles of sodium sulfide and 76.0 pound moles of sodium carbonate. The smelter gas which contained 70.9 pound moles of sulfur dioxide, 4.9 pound moles of sodium oxide and 3762.5 pound moles of a mixture of water vapor,

carbon monoxide, carbon dioxide and other inert gases was passed through an electrostatic precipitator 15:: where the solid particles of chemicals were separated from it, and the purified gas was then passed through heat exchangers to the smelter gas scrubber 23. The solid particles of chemical which are here referred to as fly ash, which were separated from the gas in the electrostatic precipitator were returned to the incoming concentrated waste liquor stream in the multi-etlect evaporator 14.

The smelt from the furnace was dissolved in a typical smelt dissolver 16 such as is used in a kraft recovery system and clarified in a kraft type clarifier 17. The clarified liquor was then partially evaporated in a crysta lizing evaporator -18 to produce a mother liquor containing 29.6 pound moles of sodium sulfide and 10 pound moles of sodium carbonate. The solid phase sodium carbonate produced in the crystallizing evaporator amounted to 66 pound moles of sodium carbonate. This solid sodium carbonate was separated from the mother iquor in the filter and washer l9 and then redissolved in dissolver tank 21. The mother liquor from the filter and washer 19 was routed to the finishing tank 26.

The sodium carbonate in the dissolver was then trans ferred to a supply tank 22 and the sodium carbonate solution from this supply tank was used as a scrubbing agent for the purified smelter gas in the smelter gas scrubber 23. Because of the very large amount of smelter gas, a recycle was provided for a portion of the efiluent solution from the scrubber, returning this recycle portion to the supply tank 22. Approximately of the sulfur dioxide was absorbed in the scrubber so that the aqueous efiluent from the scrubber contained 63.9 pound moles of sodium sulfite and 2.9 pound moles of sodium carbonate. The waste gas from the scrubber contained approximately 7.0 pound moles of sulfur in the form of sulfur dioxide and other sulfur containing compounds. The effluent solution from the scrubber was treated with .2 pound moles of sulfur dioxide in the make up absorber 25. The sulfur dioxide was prepared by burning 9.2 pound moles of sulfur with air in the sulfur burner 24. In addition to this loss of sulfur there was also an over all loss of 49 pound moles of sodium throughout the recovery cycle. The liquor efliuent from the make up absorber contained 58.9 pound moles of sodium sulfite and 14.2 pound moles of sodium acid sulfite and this eflluent was treated with 14.2 pound moles of sodium hydroxide in the finishing tank 29 in order to neutralize the sodium acid sulfite and thus form the sodium sulfite. The mother liquor from the filtering washer 14 was combined with the effiuent from the make up absorber 25 in the finishing tank after the neutralization Step. The finished liquor thus contained 73.1 pound moles of sodium sulfite, 29.6 pound moles of sodium sulfide and 10.0 pound moles of sodium carbonate. The composition of the finished liquor was therefore 73.3% by Weight sodium sulfite, 18.3% sodium sulfide and 8.4% sodium carbonate and was suitable 13 for use as fresh liquor in the Braun type pulping operation.

Example 2 A spent cooking liquor which had the same composition as the spent liquor of Example 1 was obtained from the digester, separated from the pulp and evaporated in the same manner as in Example 1. A side stream of the concentrated liquor from the evaporator 14 in an amount suificient to contain 41.8 pound moles of sulfur and 45.9 pound moles of sodium oxide was separated from the main stream of concentrated liquor leaving the multi-efiect evaporator 14. Thus the main stream of concentrated liquor entering the smelter furnace 15 contained only 58.7 pound moles of sulfur and 64.1 pound moles of sodium oxide. The furnace was a kraft type furnace, operated in the conventional manner of operating kraft type furnaces to produce a smelt having a sodium sulfide and sodium carbonate ratio of about 1 to 1. Thus the smelt from the furnace contained 29.6 moles of sodium sulfide and 29.6 moles of sodium carbonate. The side stream of concentrated black liquor was heated to about 350 F. and atomized and burned in a spray tower reactor, the walls of which were maintained at about 1400" F. The solid product from the spray tower reactor 26 contained 46.4 pound moles of sodium carbonate. This product was dissolved in the reactor dissolver 21 and thus separated from the insoluble impurities. The carbonate was further clarified in the carbonate clarifier 29 and the carbonate solution then reunited with the main stream in the supply tank 22. The wash water from the carbonate clarifier 28 was combined with the wash water of the main stream clarifier 17. The off gas from the spray tower reactor 26 had a Sulfur content equivalent to 41.8 pound moles of sulfur. In order to completely convert the sulfur content into sulfur dioxide this off gas was admitted to the smelter furnace 15 through a port in the furnace and the sulfur content of the gases was completely oxidized in the upper portion of the furnace to sulfur dioxide. The smelter gas was treated in the separator 15:: in the same manner as in Example 1 and returned to the main stream in the smelter gas scrubber 23.

The smelt from the smelter furnace 15 was dissolved in the smelt dissolver 16, clarified in the clarifier 17 and the solution obtained from the clarifier was partially evaporated in the crystallizing evaporator 18 in the same manner as the smelt was treated in Example 1. The solid sodium carbonate produced in the crystallizing evaporator 18 was separated in the filter and washer 19 from the mother liquor which contained 29.6 pound moles of sodium sulfite and 10 pound moles of sodium carbonate. The mother liquor was routed to the finishing tank 29 as in Example 1.

The solid sodium carbonate obtained from the filter and washer which amounted to 19.6 pound moles was dissolved in the carbonate dissolver 21 and this aqueous carbonate solution combined in the supply tank 22 with the carbonate solution from the carbonate dissolver 28 to form a carbonate solution containing 66.0 pound moles of sodium carbonate. It will be noted that the carbonate solution contained in the supply tank 22, the mother liquor introduced into the finishing tank and the smelter gas introduced into the smelter gas scrubber 23 all had the same composition as did the respective streams in Example 1. The balance of the operation was therefore identical with that of Example 1. The finished liquor contained 73.1 pound moles of sodium sulfite, 29.6 pound moles of sodium sulfide and 10.0 pound moles of sodium carbonate.

Example 3 The digestion in the present example was carried out with fresh cooking liquor which was identical with the liquor used in Examples 1 and 2. The operation and losses from the digester through the blow tank, brown stock washer and multi-effect evaporator were identical with those previously described in Example 1. The smelter furnace in the present example was a kraft type furnace operated in the conventional manner of operating kraft furnaces to produce a smelt containing sodium sulfide and sodium carbonate in a 1 to 1 molar ratio. The smelt from the furnace contained 52.8 pound moles of sodium sulfide and 52.8 pound moles of sodium carbonate. The smelt was dissolved in the smelt dissolver 16 and clarified in the clarifier 17 in the same manner as the smelt was treated in the previous examples. The smelter gas from the smelter furnace 15 which contained 47.7 pound moles of sulfur dioxide and 4.9 pound moles of sodium oxide was purified in a separator 15a, the solids returned to the process in the multi-effect evaporator 14, and the smelter gas conveyed to the smelter gas scrubber 23.

The aqueous solution obtained from the clarifier was split into a main stream and a side stream with the main stream containing 29.6 pound moles of sodium sulfide and 29.6 pound moles of sodium carbonate and the side stream containing 23.2 pound moles of sodium sulfide and 23.2 pound moles of sodium carbonate. The main stream thus had the same chemical constitution as did the main stream from the clarifier in Example 2. The main stream was partially evaporated in a crystallizing evaporator 18, to form solid sodium carbonate which was filtered and washed in the filter washer 119. The solid carbonate thus obtained was dissolved in the dissolver 21 in the same manner as was the main stream in Example 2. The side stream from the clarifier 17 was conveyed to a zinc cycle reactor 29 where it was reacted with a slurry containing 23.2 pound moles of zinc sulfite. The effluent from this zinc cycle reactor was passed through a zinc cycle filter 30 where the zinc sulfide was separated from the solution containing sodium sulfite and sodium carbonate. The solution from the filter which contained 23.2 pound moles of sodium sulfite and 23.2 pound moles of sodium carbonate was then conveyed to the supply tank where it is mixed with the main stream sodium carbonate solution. The zinc sulfide which was obtained from the zinc cycle filter 30 was oxidized in a Zinc cycle roaster 31 and the resultant sulfur dioxide and zinc oxide were reacted in the zinc absorber 32 to form 23.2 pound moles of Zinc sulfite which was recycled to the zinc cycle reactor 29.

The combined liquor in the supply tank contained 23.2 pound moles of sodium sulfite and 42.8 pound moles of sodium carbonate. This solution was used to scrub the purified smelter gas in the smelter gas scrubber 23. The sodium carbonate reacts with the gaseous sulfur dioxide in the scrubber so that the product solution from the scrubber contained 63.9 pound moles of sodium sulfite and 2.1 pound moles of sodium carbonate. This eifiuent solution from the smelter gas scrubber 23 was combined with additional sulfur dioxide in the make up absorber 25, and then neutralized with sodium hydroxide in the finishing tank 20 and there combined with the mother liquor from the filter and washer 19, in the same manner as in Example 2. The resultant li uor contained 73.1 pound moles of sodium sulfite, 29.6 pound moles of sodium sulfide and 10.0 pound moles of sodium carbonate and was thus identical With the finished liquor of Examples 1 and 2.

Example 4 In the present example the cooking liquor contained 73.1 pound moles of sodium sulfite, 29.6.pound moles of sodium sulfide and 14.8 pound moles of sodium carbonate. It will be noted that the inactive chemical represented by the sodium carbonate content amounted to 11.9% of the total chemical content of the fresh cooking liquor. Upon completion of the digestion the sulfur content and sodium content of the spent cooking liquor Was 102.7 pound moles of sulfur and 117.5 pound moles of sodium oxide. The digester was blown and the pulp encased l separated from the spent liquor in the brown stock washer 13 in the same manner as in the preceding example. The waste losses in the brown stock washer amounted to 2.2 pound moles of sulfur and 2.2 pound moles of sodium oxide. The black liquor was concentrated in the multieifect evaporator 14 in the same manner and to the same consistency as in the preceding example. The black liquor was then burned'in a kraft type furnace which, however, was operated to produce a smelt having a sodium sulfide to sodium carbonate ratio of 2 m1. This was accomplished by increasing the depth of the char bed and limiting the input of primary air in comparison to conventional kraft type recovery operation. The smelter gas which contained 26.9 pound moles of sulfur dioxide and 4.9 pound moles of sodium content as sodium oxide was passed through a separator where the solids were removed and recycled to the multi-eftect evaporator. The purified smelter gas was then transmitted to the smelter gas scrubber 23. The smelt from the furnace which contained 73.6 pound moles of sodium sulfide and 36.8 pound moles of sodium carbonate was dissolved in the smelt dissolver and clarified in the clarifier 17. T1 e product stream removed from the clarifier was split and a portion of the stream containing 29.6 pound moles of sodium sulfide and 14.8 pound moles of sodium carbonate was conveyed directly to the finishing tank 20. The balance of the stream which contained 44.0 pound moles of sodiurn sulfide and 22.0 pound moles of sodium carbonate was reacted in the zinc cycle reactor 29 with an aqueous slurry of zinc sulfite containing 44.0 pound moles of zinc sulfite. The zinc sulfite was thus converted into zinc sulfide and the sodium sulfide converted into sodium sulfite. The insoluble zinc sulfide was separated from the solution containing the sodium sulfide and sodium carbonate in the zinc cycle filter 30. The 44.0 pound moles of zinc sulfide was then roasted in the zinc cycle roaster 31 to form zinc oxide and sulfur dioxide and these products reacted in the zinc cycle absorber '32 to form Zinc sulfite. This zinc sulfite may be slurried with water and reused in the zinc cycle reactor 29. The sodium sulfite-sodium carbonate solution from the filter was transmitted to the supply tank 22 and used as a scrubbing medium for the purified smelter gas in the smelter gas scrubber 23 thus converting the carbonate content substantially to the sulfite. The sulfur dioxide content of the smelter gas was 26.9 pound moles as compared with 43.7 pound moles in the embodiment illustrated in FIGURES 3 and 70.9 pound moles in the embodiment illustrated in FIGURES 1 and 2. It will be noted that the crystallizing evaporator, filter and washer, and dissolver are eliminated in the present modification as compared with the embodiment illustrated by FIGURE 3. The operation of the make up absorber 25, sulfur burner 24 and finishing tank 20 are essentially the same in the present embodiment as in the preceding example illustrated in FIGURE 3. The resultant finished liquor contained 73.1 pound moles of sodium sulfite, 29.6 pound moles of sodium sulfide and 14.8 pound moles of sodium carbonate.

Now while the methods described constitute preferred embodiments of the invention it is to be understood that the invention is not limited to these embodiments and that modifications may be made without departing from the scope of the invention which is defined in the appended claims.

What is claimed is:

l. A process for recovering and reconstituting spent pulping liquor from a pulping process wherein the pulping agent is a liquor containing as active chemical 6080% sodium sulfite, 30-10% sodium sulfide and not more than about 12% inactive chemical, which consists of the steps wherein the spent liquor is separated from the pulp and concentrated by evaporating to a solids content of about 50-70%, the concentrated spent liquor is burned to produce a gaseous fraction containing sulfur dioxide and a solids fraction containing sodium sulfide and sodium carbonate in a mole ratio of sulfide to carbonate of about 1 to 2.5, the solids fraction is dissolved in water to form a sodium sulfide-sodium carbonate solution, the solution is partially evaporated to form a sulfide portion consisting of an aqueous phase containing sodium carbonate in an amount not more than about 12% 'by Weight of the chemical content or the original pulping liquor and the sodium sulfide and a solid phase containinglthe balance of the sodium carbonate, the solid and aqueous phases are separated, the sodium carbonate of the solid phase is dissolved in water to form a carbonate solution, the gaseous fraction is scrubbed with the carbonate solution to form a sulfite portion, the sulfite portion is fortified with sodium ion and sulfite ion in an amount sufficient to increase the sodium sulfite content of said sulfite portion so that the sulfur content of said portion is 6- of the sulfur content of the original pulping solution, and the sulfide and sulfite portions are combined to form a reconstituted cooking liquor.

2. A process for recovering and reconstituting spent pulping liquor from a pulping process wherein the pulping agent is a liquor containing as active chemical 68 80% sodium sulfite, 30l0% sodium sulfide and not more than about 12% inactive chemical, which consists of the steps wherein the spent liquor is separated from the pulp and concentrated by evaporating to a solids content of about 50-70%, the concentrated spent liquor is separated into a major part containing about 0% of the chemical content of the liquor and a minor part con taining about 40% of the chemical content of the liquor, the major part of the concentrated liquor is burned "to form a gaseous fraction containing sulfur dioxide and a solids fraction containing sodium sulfide and sodium carbonate and having a molar ratio of sodium sulfide to sodium carbonate of about 1 to l, the minor part is pyrolyzed to form a sulfur containing gas and a sodium carbonate fraction, the sulfur containing gas is further oxidized to sulfur dioxide and combined with the gaseous fraction, the solids fraction of the major part is dissolved in water to form a sodium sulfide sodium carbonate solution, the solution is partially evaporated to form a sulfide portion consisting of an aqueous phase containing sodium carbonate in an amount not more than 12% by weight of the chemical content of the original pulp liquor and the sodium sulfide and a solid phase containing the balance of the sodium carbonate, the solid and liquod phase are separated, the sodium carbonate of the solid phase and the sodium carbonate fraction of the minor part are dissolved and combined to form a sodium carbonate solution, the gaseous fraction is scrubbed with the carbonate solution to form a sulfite portion, the sulfite portion is fortified with sodium ion and sulfite ion in an amount sufiicient to increase the sodium sulfite content of said sulfite portion so that the sulfur content of said portion is 6080% of the sulfur content of the original pulping solution, and the sulfide and sulfite portions are combined to form a reconstituted cooking liquor.

3. A process for recovering and reconstituting spent pulping liquor from a pulping process wherein the pulping agent is a liquor containing as active chemical 60-- 80% sodium sulfite, 30-10% sodium sulfide and not more than about 12% inactive chemical, which consists of the steps wherein the spent liquor is separated from the pulp and concentrated by evaporating to a solids content of about 5070%, the concentrated spent liquor is burned to produce a gaseous fraction containing sulfur dioxide and a solids fraction containing sodium sulfide and sodium carbonate and having a mole ratio of sulfide to carbonate of about 1 to 1, the solids fraction is dissolved in water to form a smelt solution, the smelt solution is separated into a main stream containing about 55% of the chemical content of the smelt solution and a side stream contain-- ing about 45% of the chemical content of the smelt solution, the main stream is partially evaporated to form a sulfide portion consisting of an aqueous phase containing not more than about 12% by weight of the chemical content of the original pulping liquor of sodium carbonate and the sodium sulfide and a solid phase containing sodium carbonate, the solid phase is dissolved in Water to form a sodium carbonate solution, the side stream is reacted with zinc sulfide to form a sodium sulfite-sodium carbonate solution and solid zinc sulfide, the solid zinc sulfide is separated from the solution, the sodium sulfitesodium carbonate solution is combined With the sodium carbonate solution to form a combined solution, the gaseous fraction is scrubbed with the combined solution to form a sulfite portion, the sulfite portion is fortified with sodium ion and sulfite ion in an amount sufiicient to increase the sodium sulfite content of said sulfite portion so that the sulfur content of said portion is 6080-% of the sulfur content of the original pulping solution, and the sulfide and sulfite portions are combined to form a recon stituted cooking liquor.

4. A process for recovering and reconstituting spent pulping liquor from a pulping process wherein the pulping agent is a liquor containing as active chemical 60- 80% sodium sulfite, 30-10% sodium sulfide and not more than about 12% inactive chemical, which consists of the steps wherein the spent liquor is separated from the pulp and concentrated by evaporating to a solids content of about 50-70% the concentrated spent liquor is burned to produce a gaseous fraction containing sulfur dioxide and a solids fraction containing sodium sulfide and sodium carbonate in a molar ratio of sulfide to carbonate of about 2 to 1, the solids fraction is dissolved in water to form a smelt solution, the smelt solution is separated into a main stream containing about 60% of the chemical content of the smelt stream and a side stream containing about of the chemical content of the stream, the main stream is reacted with zinc sulfite to form a sodium sulfite-sodium carbonate solution and a zinc sulfide precipitate, the zinc sulfide precipitate is separated from the sodium sulfite-sodium carbonate solution, the gaseous fraction is scrubbed with the sodium sulfite-sodium carbonate solution to form a sulfite portion, the sulfite portion is fortified with sodium ion and sulfite ion in an amount sufiicient to increase the sodium sulfite content of said sulfite portion so that the sulfur content of said portion is -80% of the sulfur content of the original pulping solution, and combining the sulfide and sulfite portions to form a reconstituted cooking liquor.

5. The process of claim 3 wherein the zinc sulfide is reconverted to zinc sulfite by first roasting the zinc sulfide with air to form solid zinc oxide and gaseous sulfur dioxide, suspending the Zinc oxide in water to form a Zinc oxide slurry, reacting the zinc oxide slurry with the gaseous sulfur dioxide to form an aqueous zinc sulfite slurry, and returning the converted zinc sulfite slurry to the zinc sulfite reactor.

References Cited in the file of this patent UNITED STATES PATENTS 1,387,441 Braun Aug. 9, 1921 1,843,704 Bradley Feb. 2, 1932 2,792,350 Bradley et a1 May 14, 1957 2,802,791 Whitney et a1 Aug. 13, 1957 2,841,561 Gray et al. July 1, 1958 2,862,887 Boyer Dec. 2, 1958 2,898,994 Thomsen Aug. 11, 1959 

1. A PROCESS FOR RECOVERING AND RECONSTITUTING SPENT PULPING LIQUOR FROM A PULPING PROCESS WHEREIN THE PULPING AGENT IS A LIQUOR CONTAINING AS ACTIVE CHEMICAL 60-80% SODIUM SULFITE, 30-10% SODIUM SULFIDE AND NOT MORE THAN ABOUT 12% INACTIVE CHEMICAL, WHICH CONSISTS OF THE STEPS WHEREIN THE SPENT LIQUOR IS SEPARATED FROM THE PULP AND CONCENTRATED BY EVAPORATING TO A SOLIDS CONTENT OF ABOUT 50-70%, THE CONCENTRATED SPENT LIQUOR IS BURNED TO PRODUCE A GASEOUS FRACTION CONTAINING SULFUR DIOXIDE AND A SOLIDS FRACTION CONTAININGSODIUM SULFIDE AND SODIUM CARBONATE IN A MOLE RATIO OF SULFIDE TO CARBONATE OF ABOUT 1 TO 2.5, THE SOLIDS FRACTION IS DISSOLVED IN WATER TO FORM A SODIUM SULFIDE-SODIUM CARBONATE SOLUTION, THE SOLUTION IS PARTIALLY EVAPORATED TO FORM A SULFIDE PORTION CONSISTING OF AN AQUEOUS PHASE CONTAINING SODIUM CARBONATE IN AN AMOUNT NOT MORE THAN ABOUT 12% BY WEIGHT OF THE CHEMICAL CONTENT OF THE ORIGINAL PULPING LIQUOR AND THE SODIUM SULFIDE AND A SOLID PHASE CONTAINING THE BALANCE OF THE SODIUM CARBONATE, THE SOLID AND AQUEOUS PHASES ARE SEPARATED, THE SODIUM CARBONATE OF THE SOLID PHASE IS DISSOLVED IN WATER TO FORM A CARBONATE SOLUTION, THE GASEOUS FRACTION IS SCRUBBED WITH THE CARBONATE SOLUTION TO FORM A SULFITE PORTION, THE SULFITE PORTION IS FORTIFIED WITH SODIUM ION AND SULFITE ION IN AN AMOUNT SUFFICIENT TO INCREASE THE SODIUM SULFITE CONTENT OF SAID SULFITE PORTION SO THAT THE SULFUR CONTENT OF SAID PORTION IS 6080% OF THE SULFUR CONTENT OF THE ORIGINAL PULPING SOLUTION, AND THE SULFIDE AND SULFITE PORTIONS ARE COMBINED TO FORM A RECONSTITUTED COOKING LIQUOR. 